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AR# 8187: Virtex-E, Spartan-IIE LVDS/LVPECL - How do I use LVDS/LVPECL I/O Standards?
Virtex-E, Spartan-IIE LVDS/LVPECL - How do I use LVDS/LVPECL I/O Standards?
How should one design Virtex-E/Spartan-IIE LVDS or LVPECL I/Os?
When using LVDS IOSTANDARD, a UCF or NCF file with complete pinLOC information must be created to ensure that the I/O banking rules are not violated. If a UCF or NCF file is not used, PAR will issue errors. There are two ways to use the LVDS IOSTANDARD:
1. Use the IOSTANDARD attribute in the UCF file. 2. Instantiate LVDS input and output buffers.
LVDS Input buffer:
An LVDS input buffer may be placed in a wide number of IOB locations. The exact locations are dependent on the package that is used. The Virtex-E/Spartan-IIE package information lists the possible locations as IO_L#P for the P-side, and IO_L#N for the N-side, (where "#" is the pair number).
Using the UCF file:
The following syntax can be used in the UCF file to set the IOSTANDARD as LVDS:
NET <input net name> IOSTANDARD = LVDS; NET <input net name> LOC=P(pin name);
The above is an example for just one LVDS input pin. In a real design, users must lock down all the I/O pins in the UCF file. Otherwise, PAR will issue errors.
Only one input buffer is required to be instantiated in the design and placed on the correct IO_L#P location. The N-side of the buffer will be reserved, and no other IOB is allowed to be placed on this location.
In the physical device, a configuration option is enabled that routes the pad wire from the IO_L#N IOB to the differential input buffer, located in the IO_L#P IOB. The output of this buffer then drives the output of the IO_L#P cell or the input register in the IO_L#P IOB. In FPGA Editor, it will appear that the second buffer is unused. However, any attempt to use this location for another purpose will cause a DRC error in the software.
-- VHDL instantiation: data0_p : IBUF_LVDS port map (I=>data(0), O=>data_int(0));
All LVDS buffers must be explicitly placed on a device. For the input buffers, this may be done with the following constraint in the ".ucf" or ".ncf" file.
NET data<0> LOC = D28; # IO_L0P
Creating a LVDS Output Buffer
An LVDS output buffer may be placed in a wide number of IOB locations. The exact locations are dependent upon the package that is used. The Virtex-E/Spartan-IIE package information lists the possible locations as IO_L#P for the P-side, and IO_L#N for the N-side (where "#" is the pair number).
Using a UCF file:
The following syntax can be used in the UCF file to specify the LVDS IOSTANDARD for outputs.
Note: Both outputs must be specified in the UCF file and locked down to positive and negative output pins.
NET <output pad net name_positive> IOSTANDARD=LVDS; NET <output pad net name_negative> IOSTANDARD=LVDS; NET <output pad net name_positive> LOC=P22; NET <output pad net name_negative> LOC=P23;
The above is an example of just one pair of LVDS outputs. In a real design, users must lock down all the I/O pins. Otherwise, PAR will issue errors.
Both output buffers are required to be instantiated in the design and placed on the correct IO_L#P and IO_L#N locations. In addition, the output (O) pins must be inverted with respect to each other (one HIGH and one LOW). Failure to follow these rules will lead to DRC errors in the software.
-- VHDL instantiation
data0_p : OBUF_LVDS port map (I=>data_int(0), O=>data_p(0)); data0_inv: INV port map (I=>data_int(0), O=>data_n_int(0)); data0_n : OBUF_LVDS port map (I=>data_n_int(0), O=>data_n(0));
All LVDS buffers must be explicitly placed on a device. For output buffers, this may be done with the following constraint in the ".ucf" or ".ncf" file:
NET data_p<0> LOC = D28; # IO_L0P NET data_n<0> LOC = B29; # IO_L0N
Synchronous vs. Asynchronous Outputs
If the outputs are synchronous (registered in the IOB), then any IO_L#P|N pair can be used. If the outputs are asynchronous (no output register), you must use one of the pairs that is part of the same IOB group at the end of a ROW or COLUMN in the device.
The LVDS pairs that can be used as asynchronous outputs are listed in the Virtex-E or Spartan-IIE pin-out tables. Some pairs are marked as asynchronous-capable for all devices in that package, and others are marked as available only for that specific device in the package. If the device size might be changed at some point in the product lifetime, then only the common pairs for all packages should be used.
Adding an Output Register (synchronous output)
All LVDS buffers may have an output register in the IOB. The output registers must be in both the P-side and N-side IOBs. All the normal IOB register options are available (FD, FDE, FDC, FDCE, FDP, FDPE, FDR, FDRE, FDS, FDSE, LD, LDE, LDC, LDCE, LDP, LDPE). The register elements may be inferred or explicitly instantiated in the HDL code.
Special care must be taken to insure that the D pins of the registers are inverted, and that the INIT states of the registers are opposite each other. The clock pin (C), clock enable (CE) and set/reset (CLR/PRE or S/R) pins must connect to the same source. Failure to do this will lead to a DRC error in the software.
The register elements may be packed in the IOB using the IOB property "TRUE" on the register, or by using the "map -pr [i|o|b]" (where "i" is inputs only, "o" is outputs only, and "b" is both inputs and outputs).
The following is an example of VHDL code using LVDS registered outputs and LVDS input "a", targeting v100epq240. It uses both the LVDS instantiation and UCF IOSTANDARD attribute:
library ieee; use ieee.std_logic_1164.all;
entity mux is
port (a,b,c,d,e,f,g,h, clk, reset : in std_logic; S : in std_logic_vector (2 downto 0); q, q_n : out std_logic); end entity;
architecture mux_arch of mux is
component OBUF_LVDS port ( I : in std_logic; O : out std_logic); end component;
signal a_int, q_int, mux_out, mux_out_n, muxout_n_reg : std_logic;
process (a,b,c,d,e,f,g,h,s) begin
case S is when "000" => q_int<=a; when "001"=> q_int<=b; when "010" => q_int<=c; when "011" => q_int<=d; when "100" => q_int<=e; when "101" => q_int<=f; when "110" => q_int<=g; when "111" => q_int<=h; when others => q_int<='X'; end case; end process;
process (clk, reset) begin if (reset='1') then mux_out<='0'; elsif (clk'event and clk='1') then mux_out<=q_int; end if; end process; mux_out_n <= not q_int;
process (clk, reset) begin if (reset='1') then muxout_n_reg<='1'; elsif (clk'event and clk='1') then muxout_n_reg<=mux_out_n; end if; end process;
U1 : OBUF_LVDS port map(I=>mux_out, O=>q); U2 : OBUF_LVDS port map(I=>muxout_n_reg, O=>q_n);
Here are the UCF constraints:
net q LOC=P20; net q_n loc=P21; net a IOSTANDARD=LVDS; net a LOC=P27; net b LOC=P168; net c LOC=P167; net d LOC=p163; net e LOC=p162; net f loc=p161; net g loc=p160; net h loc=p159; net clk loc=p210; net reset loc=P157; net s(0) loc=p156; net s(1) loc=p155; net s(2) loc=p154;
Make sure map -pr o option is used to ensure that the FFs are being pushed inside the I/O block.
Designing with the LVPECL IOSTANDARD is the same as designing with LVDS. Users must instantiate the I/O buffer names in the HDL codes.